1. Field of the Invention
This invention generally relates to an electric power steering apparatus in which the force required for operating the steering wheel is assisted with the turning force of an electric motor and, more in particular, to a power steering apparatus with a miniaturized electric motor and a reduction device.
2. Description of the Prior Art
There has been developed an electric power steering apparatus in which a motor for assisting the steering operation is driven on the basis of the detected results of the steering torque applied on the steering wheel and the force required for operating the steering wheel of a car is assisted with the turning force of the motor to make the driver feel comfortable in steering the car.
Conventional power steering apparatus comprises a rack shaft, extending from side to side of the car body, with each of both end portions thereof connected to each of the wheel on either side of the body by means of a separate tie rod, and a pinion engaging with said rack shaft in the middle part thereof and connected interlockingly to the steering wheel. In a car equipped with a rack-pinion type steering mechanism in which the rotation of the pinion caused by the rotational operation of the steering wheel is converted to the displacement of the rack shaft in the longitudinal direction thereof to carry out steering, the power steering mechanism is classified into two types according to the position where the motor for assisting steering is disposed. That is, one is such that the aforesaid pinion shaft is extended further from the place where this engages with the rack shaft and the aforesaid motor is disposed to transmit the turning force on the extended portion through a proper reduction device. The other is such that an auxiliary pinion is provided engaging with the rack shaft at a place in the longitudinal different from that of the place where the pinion engages with the rack shaft and said motor is disposed to transmit the turning force thereof to said auxiliary pinion through a proper reduction device. The former is called one-pinion type and the latter two-pinion type according to the number of pinions engaging with the rack shaft.
On the other hand, a torque sensor is generally used as means for detecting the steering torque as relative displacement between said two shafts caused by torsions of the torsion bar as mentioned above in which said pinion shaft is firstly divided into two in the middle part thereof and these divided shafts are connected to each other on the same axis through a torsion bar, then a potentiometer is constructed around this connecting part.
In this way every type of the power steering apparatus as mentioned above has adopted a construction in which the turning force of the motor for assisting steering is transmitted to the pinion which engages with the rack shaft from the rectangularly or obliquely crossing direction. In such a construction said motor is provided around the rack shaft in the steering mechanism. This type of construction, however, has such a defect that the positioning for disposing the steering mechanism is restricted, for the mechanism is provided at a lower part of the engine room with a small excess space. Besides, as mentioned above, it is necessary to provide a reduction device having a great reduction ratio between the motor and the pinion in order to reduce the rotational speed of the motor to a traveling speed of the rack shaft suitable for steering and in order to let the rack shaft have sufficient assisting power. This reduction device is massive, so that it is difficult to dispose the motor as mentioned above in the mechanism. Consequently, when the manufacturer desires to mount such a power steering device in the car, sometimes it has been necessary to draw newly a design of the engine room so as to secure the space for disposing the motor and reduction device.
Besides, in a power steering apparatus of such a construction, while the positioning for disposing the pinion shaft in the direction from side to side of the car body is limited by the relation with the rack shaft disposed at the central part of the car body on the one hand, the positioning for disposing the steering wheel which is interlockingly connected to the pinion shaft is limited by the relation with the driver's seat in the compartment on the other hand, so that the pinion shaft and the rack shaft are forced to be disposed asymmetrically and the pinion shaft and the rack shaft are engaged with each other obliquely to intersect their axis for the purpose of connecting them to each other without strain. Consequently, a considerable amount of load is applied on the pinion shaft at a place where it engages with said rack shaft. On the other hand, in a torque sensor in which a potentiometer is constructed on the pinion shaft, as mentioned above, picking-out of the output signal of the sensor and the impression of prescribed electric potential on the sensor are carried out through a slip ring because of the torque sensor rotating together with the pinion shaft, so that when the pinion shaft has carried out displacement in the longitudinal direction thereof due to the load caused in the way as mentioned above, the torque sensor will often misoperates as a result of the change of the position where the slip ring and the detecting wire slidingly contacts to each other even if said amount of displacement is slight one, and the motor for assisting steering is driven in accordance to the erroneously-detected result, which causes discrepancies between the amount of steering operation and that of the actual steering and further causes such a trouble that the detecting wire is deviated from the position where the wire slidingly contacts to the slip ring, when the amount of displacement is great, which results in that detection of the steering torque comes to be impossible.
Therefore, in a conventional power steering apparatus, the pinion shaft is held by means of a pair of angular ball bearings disposed on both sides of the place where the pinion shaft engages with the rack shaft to let these bearings bear the load in the longitudinal direction. In addition, in assembling the pinion shaft housing to the rack shaft housing, the angular ball bearings is constructed to be affected by prescribed pre-pressurization through separate adjustment of the thickness and/or number of shims provided therebetween in order to make the displacement in the longitudinal direction as little as possible. This type of steering apparatus has such defects that it requires much time and labor in assembling and that alien substances in the rack shaft housing such as grease and the like sealed in the place where the pinion shaft and the rack shaft engages each other will intrude in the place where the torque sensor is provided and adhere to the sensor, which has often caused misoperatons of the torque sensor and eventually discrepancies between the amount of steering operation and that of the actual steering.
Not only a one-pinion type power steering apparatus as mentioned above, but also a two-pinion type power steering apparatus wherein the turning force of the motor for assisting the steering is transmitted to auxiliary pinion shaft engaging with said rack shaft at a place in the longitudinal direction different from the pinion shaft has had such defects as mentioned above. Furthermore, such troubles have occurred in the power steering apparatus constructed in the steering mechanism of a type other than the rack-pinion type as mentioned above, such as ball-nut type steering mechanism and the like.